This application relates to a process and an apparatus for coating a printed circuit board and the resultant board, and more particularly to a process and an apparatus for coating a printed circuit board by (1) securing the board in the horizontal position in a coating chamber, and (2) coating the board by flooding the chamber with a coating solution while applying an electric charge to the printed circuit board.
Where an electric circuit formed by wires has been used in many situations in the past, it is now known that such wiring can cause problems. The printed circuit board can replace the wired circuit and solve these problems. Also in this fashion, the structure of an electrical item is simplified with the use of a printed circuit board.
The raw material for a printed circuit board is a non conductive substrate coated with a conductor or conductors. Generally such substrates are flat fiber glass materials. The first conductive coating applied is copper. Copper may be applied by electroplating, lamination or other suitable fashion.
The copper coated substrates are then covered with a photoemulsion. This photoemulsion permits photographic development of the desired circuitry to expose the copper on the fiberglass board. The photoemulsion is exposed photographically. After exposure, the exposed photoemulsion is washed away as desired, and the copper thereunder is exposed. At that point, the exposed copper can be coated if the circuit is a photographic positive, or etched away if the circuit is a photographic negative.
Assuming a photographic positive, the copper, as desired, can be coated with nickel, gold, or other metals or conductive materials as the next step or steps in making the printed circuit board. Then the remainder of the photoemulsion may be washed away from the copper. To complete the printed circuit board, the copper that is unprotected by nickel and gold--or other suitable metals or conductive material--is removed. Thus, after the exposed copper is coated with nickel and gold, it is possible to rinse off the photoemulsion and etch off the uncoated copper to complete the board.
Assuming a photographic negative, the copper, as exposed, can be etched or otherwise removed from the substrate. Then the remainder of the photoemulsion may be washed away to expose the remaining copper. To complete the printed circuit board, the copper that remains is coated by nickel and gold--or other suitable metals or conductive material.
A plurality of printed circuit boards may be stacked for a desired purpose. When stacked each printed circuit board may be interconnected by apertures in the board or boards. A drill pattern is established for the apertures in the printed circuit board by photoimagery. It is critical to coat the interior of the aperture in the board properly, since the aperture provides the connection from one side of the board to the other or from one circuit to the other. Such coating of the aperture is difficult--especially when reproducibility and uniformity of the board is required for mass production techniques.
It is important to maintain a tight control of the criticality of the coating of the interior portion of the aperture. This is especially true when there is a sandwich or plurality of printed circuit boards available to use. This plurality board increases the size of the printed circuit board sandwich. As the size of the printed circuit board and therefore the number of available circuits increase, the printed circuit boards can be used to replace more wires in a circuit.
The trend and goal in printed circuit boards is to provide a larger board for more circuits with smaller apertures for connecting the circuits. The apertures used to be in the range of two hundred thousandths (0.200 inch) of an inch, which is about five and eight hundredths (5.08) millimeter. (A thousandth of an inch or 0.001 inch is 0.0254 millimeter. One micron is equal one millionth of a meter.) However now the apertures are down to thirty thousandths (0.030 inch) of an inch, which is about 0.762 millimeter and getting smaller.
In a particular case, the aperture has a diameter of eighteen thousandths (0.018 inch or 0.457 millimeter) of an inch. A copper coating on the interior of the aperture reduces the diameter of the hole to fifteen thousandths (0.015 inch or 0.381 millimeter) of an inch. On the copper is desired to put up to two hundred millionths (0.000200 inch) of an inch of nickel and at least fifty millionths (0.000050 inch or 0.00127 millimeter or 1.27 microns) of an inch of gold.
With these smaller apertures it has become extremely difficult to provide the metal coating on the interior thereof. An attempt is made to meet these difficulties with a combination of chemistry and electroplating. Chemical changes involve adjusting the viscosity of the electroplating solution and other actions. However, adjusting the viscosity of solutions and changing that particular coating method destroys the integrity of the solution and makes it extremely difficult to monitor the integrity to achieve the desired coating. Accordingly, it is desired to avoid changes in the coating solution.
It is customary to coat the plates in a vertical fashion in the coating bath. Various methods have been used to achieve equilibrium of the coating. However, due to the depth of solution and the differing pressures on the various levels of the board due to the force of the liquid, equal coating contact of the apertures of the apertures with the coating solution is not possible. The unequal pressure due to the differing depths of the liquid at various points on a vertical board causes problems that make reproducibility and uniformity almost impossible.
In the field of solid state printed circuit boards, it is highly desirable to coat within an aperture on the board. It is desired to provide a minimized aperture to surface coating ratio while coating the circuit board to provide various connections and support for the devices. It is also sometimes desirable to stack a number of boards in an appropriate relationship. The stacking of the boards results in a requirement that the interior of apertures throughout a stack of printed circuit boards be coated.
Another possible solution is to move the board to be coated between a spray of the solution. Adjusting of the spray to uniformity combined with positioning the board properly is difficult.
In the manufacture of printed circuit boards, designers are moving toward smaller apertures and thicker boards. For example the aperture sizes are now down to about four thousandths (0.004 inch or 0.1016 millimeter) of an inch and the board thickness is about two hundred fifty thousandths (0.250 inch or 6.35 millimeters) of an inch. The acronym or buzz word used in this matter is HARP, High Aspect Ratio Plating. While the demand for these HARP products is very high, use is limited to research and development prototypes and very low volumes--due to the inability to produce the desired coatings efficiently. Even the Japanese are quite limited in their success with this type of coating. Today's technology is not adequate in equipment or coating solutions or other chemistry to achieve the desired coating results.
This especially desired high aspect ratio plating is critical. The ratio is determined by measuring the diameter of the hole to the board thickness. The lower this ratio, the more difficult it is to coat the interior of the hole. Thus, the failure to coat the hole properly renders the board inoperable. It is highly desired to coat the board and the interior of the aperture in a suitable fashion.
Various attempts are made to achieve this desired coating. One common attempt involves adjusting the vicosity of the liquid. However, viscosity adjustments can only take the coating only so far. With the viscosity adjustments, the coating process and solution often lose the effectiveness of the coating. It is desired that an adequate supply of fresh solution for coating the board should be applied uniformly and with equal pressure.
The application of the desired coatings in the prior art manner is proven difficult, with the coatings having insufficient uniformity to comply with the desired results. It is therefore highly desired to achieve a coating process and resultant article within the desired parameters of thickness and uniformity with reproducibility.